As is known in the art, the “compression ratio” of an internal combustion engine is defined as the ratio of the cylinder volume when the piston is at bottom-dead-center (BDC) to the cylinder volume when the piston is at top-dead-center (TDC)—generally, the higher the compression ratio, the higher the thermal efficiency and fuel economy of the internal combustion engine. Unfortunately, compression ratios are limited by the availability of high-octane fuels needed to prevent combustion detonation or knock at high engine loads, and therefore a compression ratio is selected to operate on available fuels, and avoid knock. So-called “variable compression ratio” internal combustion engines have been developed, for example, having higher compression ratios during low load conditions and lower compression ratios during high load conditions.
In an engine with a variable compression ratio mechanism, the engine compression ratio can be selected to achieve the best fuel economy of a vehicle. However, drivability and engine knock issues may occur by changing engine compression ratio while driving a vehicle in different environmental conditions. To ensure the switching of compression ratio happens with minimum knock and as smooth as possible at every possible real-world driving condition, not only must the engine operating conditions be taken into consideration but also environmental conditions have to be taken into considered in the compression ratio selection. The problem is how to take into account those factors so as to select appropriate engine compression ratio to obtain optimum fuel economy without sacrificing drivability.
In one variable ratio internal compression ratio system, the Variable Compression Ratio (VCR) mechanism does not allow the engine to change Compression Ratio (CR) when engine speed is greater than a certain limit (this limit is referred to herein as compression ratio switching engine speed limit). More particularly, the CR change is only possible either at intake or exhaust stroke. Therefore, for such VCR mechanism to execute CR switching, certain time duration of intake or exhaust time period is required. However, as the engine speed increases, the time that a cylinder stays on either intake or exhaust stroke gets smaller, explaining why the VCR mechanism is not capable of switching from one CR to the other CR at higher engine speed. When the VCR engine loses an opportunity to switch to low compression mode at a higher engine speed, it may result in severe engine knock at higher engine load and speed, possibly resulting in engine damage.
One of the possible and practical solutions for this problem is to switch to low compression mode in advance when the engine speed is projected to exceed the compression ratio switching engine speed limit.
In accordance with the invention, a method is provided for operating an internal combustion engine comprising selecting a compression ratio for the engine as a function of a projected engine speed.
In accordance with the present invention, the system predicts whether the engine speed may exceed the compression ratio switching engine speed limit.
In accordance with the invention, a method is provided for operating an internal combustion engine. The method includes: providing a functional relationship between time rate of change in engine speed, and compression ratio switching engine speed limit; determining time rate of change in engine speed; determining from the determined rate of change of engine speed and the function whether the engine speed exceeds the compression ratio switching engine speed; and commanding the engine to operate at a relatively low compression ratio if the determined time of change in engine speed exceeds the compression ratio switching engine speed limit and commanding the engine to operate at a relatively high compression ratio if the determined time of change in engine speed is less than the compression ratio switching engine speed limit.
In one embodiment, the prediction is a function of the derivative of engine speed (i.e., the time rate of change in engine speed, d[engine_speed]/dt), which is calculated at each time the engine speed is sampled in the Engine Control Module (ECM). This derivative of engine speed indicates whether the engine speed was increasing or decreasing during last sampling period (i.e., positive derivative number indicates engine speed was increasing and negative means engine speed is decreasing).
In one embodiment, a method is provided for operating an internal combustion engine. The method includes providing a function relating time rate of change in engine speed and a compression ratio switching engine speed limit. The compression ratio switching engine speed limit is related to the engine speed at which to initiate compression ratio switching. The engine is operated with a compression ratio selected in accordance with engine operating conditions independent of a time rate of change in engine speed. A time rate of change in engine speed is determined during the engine operation. The method determines from the determined time rate of change in engine speed, the compression ratio switching engine speed limit. The engine is commanded to operate at a relatively low compression ratio if the engine speed exceeds the compression ratio switching engine speed limit; otherwise, the engine continues to operate with a compression ratio selected in accordance with engine operating conditions independent of a time rate of change in engine speed.
In one embodiment, to reduce the effect of signal noise generation which may result from using the derivative of engine speed, depending on the engine inertia or rate of throttle manipulation, the system includes a filter for filtering engine speed derivative, for example, with a software filter. With such filtering, smooth engine speed trends can be obtained (again, positive indicating engine speed increment and negative indicating engine speed reduction without too much of signal noise.
In one embodiment, the filtered engine speed derivative and the table is a two-dimensional (2-D) function. The method uses the filtered derivative with the 2 D look-up function threshold to determine if the engine speed is going to exceed the compression ratio switching engine speed limit or not. Current rate of change of engine speed is used as an independent variable of this 2D threshold table so that it can be calibrated with different threshold at different engine speed.
The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.